The present invention relates to a polarization mode dispersion measuring method and a polarization mode dispersion measuring system, and relates, particularly, to a polarization mode dispersion measuring method and a polarization mode dispersion measuring system employing a technique for measuring in high precision the polarization mode dispersion of an optical device as a measured object.
As known, among optical devices, refractive indexes of an optical fiber, for example, vary depending on the direction of polarization.
Because of differences in refractive indexes due to this difference in polarization direction, a group delay time difference occurs in the polarization directions of light transmitted by this optical fiber.
As a result, there occurs a phenomenon that when an optical pulse or the like is incident to an input end of the optical fiber, the width of the optical pulse is expanded at an output end.
This phenomenon is called xe2x80x9cpolarization mode dispersionxe2x80x9d (PMD), and, in a long-distance and high-speed optical transmission system, this is an important factor determining the performance of this system.
Particularly, in the case of carrying out an optical transmission of 1.55 xcexcm by using a 1.3 xcexcm zero-dispersion optical fiber, in mainstream use throughout the world at present, or in a wavelength-multiplexed system and a high-speed transmission (Gbit/s order) long-distance optical submarine cable system that use an optical amplifier, this polarization mode dispersion (PMD) becomes a big problem, and extremely limits the propagation distance.
Therefore, in the case of designing an optical transmission system or the like, it is necessary to measure in advance a level of the polarization mode dispersion (PMD) of optical devices including an optical fiber that are used in this system.
As a method for measuring this polarization mode dispersion (PMD), a fixed analyzer method that has a characteristic of high measuring precision while having a simple structure has been widely used among an interference method in a time region, the fixed analyzer method in a frequency region, and a polarization analysis method that have been conventionally known.
FIG. 6 shows a conventional measuring system for measuring a polarization mode dispersion by using this fixed analyzer method.
According to this measuring system, after a linearly polarized beam in a specific polarization direction has been extracted by a first polarizer 12 from a light emitted from a broad band light source 11, this extracted linearly polarized beam is incident to one end of a measured object 1.
Then, from a light emitted from the other end side of the measured object 1, a linearly polarized beam in the same polarization direction as that of the light from the first polarizer 12 is extracted by a second polarizer 13, and thereafter, this extracted linearly polarized beam is incident to an optical spectrum analyzer 14.
In this case, a polarization direction of the linearly polarized beam incident to the measured object 1 is set to have an angle of 45 degrees with respect to the X axis (or the Y axis) by the first polarizer 12, when the incident surface of the measured object 1 is the XY plane.
Further, the second polarizer 13 is also matched with the direction of this first polarizer 12.
The transmission speed of an X-axis component and the transmission speed of a Y-axis component of the linearly polarized beam that has been incident to the measured object 1 do not become the same, due to a difference between the refractive index of a portion of the measured object 1 that follows the X-axis and the refractive index of a portion that follows the Y-axis.
For example, as shown in FIG. 7A, assume that a linearly polarized beam of a certain wavelength xcexa is incident to the measured object 1, and its Y-axis component is delayed (or advanced) by 2 xcfx80 from the X-axis component. Then, the polarization direction of the wavelength xcexa emitted from the measured object 1 becomes the same as the incident status.
Accordingly, the light of this wavelength xcexa is transmitted through the second polarizer 13 with a small loss.
Further, as shown in FIG. 7B, assume that a linearly polarized beam of a certain wavelength xcexb is incident to the measured object 1, and its Y-axis component is delayed (or advanced) by xcfx80 from the X-axis component. Then, the polarization direction of the wavelength xcexb emitted from the measured object 1 becomes orthogonal with the polarization direction extracted by the second polarizer 13.
Accordingly, the light of this wavelength xcexa is attenuated large by the second polarizer 13 and cannot substantially be transmitted through.
This phenomenon occurs due to a difference in the delay time attributable to a difference between the refractive indexes of the measured object in the X-axis direction and Y-axis direction.
Further, as this delay time difference has continuity in the wavelength of a transmitted light, the intensity of the light transmitted through the second polarizer 13 changes in a constant period with respect to a change in the wavelength.
As a result, the optical spectrum analyzer 14 displays a spectrum waveform of which level changes periodically, as shown in FIG. 8.
Then, according to this fixed analyzer method, a differential group delay time that shows a level of PMD (this is called a PMD value) is calculated by the following equation, using a first peak wavelength xcex1 and a last peak wavelength xcex2, based on the assumption that a wavelength distance between adjacent peaks (or between adjacent bottoms) of this spectrum waveform is expressed as a phase difference 2xcfx80:
xcex94xcfx84=k(nxe2x88x921)xc2x7xcex1xc2x7xcex2/(Cxc2x7xcex94xcex) 
where, k represents a mode coupling coefficient, C represents an optical speed, n represents a number of peaks, and xcex94xcex=xcex2xe2x88x92xcex1.
The mode coupling coefficient k is a value equal to or lower than 1 that is determined according to an inter-mode coupling status between the X-axis component and the Y-axis component of the light transmitted through the measured object 1. When the optical path length is not very long, it is possible to set k=1.
However, according to the above-described conventional PMD measuring system, based on the principle of measurement, it is not possible to calculate a PMD value, when at least two peaks do not exist in the spectrum displayed in the optical spectrum analyzer 14.
In other words, in the above-described equation for calculating a PMD value, it is a measurement limit of a PMD value when xcex94xcex is a maximum at the time of n=2, that is, up to when two peaks (or bottoms) exist within a bandwidth of a broad band light source.
Therefore, according to the above-described conventional PMD measuring system, the measurement limit of a PMD value is controlled by the bandwidth of the broad band light source.
Actually, a bandwidth (half-value width) that can be used in the fixed analyzer method is up to about 200 nm, and a measurement limit of a PMD in the 1500 nm band becomes xcex94xcfx84=50xc3x9710xe2x88x9212 (second).
Therefore, in order to obtain a PMD value equal to or smaller than this by using the fixed analyzer method, there is a problem that it is necessary to use other methods having complex structures (the polarization analysis method in a frequency region, and the interference method in a time region).
An object of the present invention is to provide a polarization mode dispersion measuring method and a polarization mode dispersion measuring system capable of obtaining a PMD value up to a smaller measurement limit, in a simple structure according to the fixed analyzer method, by solving the above conventional problems.
In order to achieve the above object, according to the present invention,
(1) there is provided a polarization mode dispersion measuring method comprising the steps of:
applying a polarization mode dispersion to a linearly polarized beam extracted from a light having a substantially uniform spectrum, with a reference object having a predetermined polarization mode dispersion value;
making the light applied with the predetermined polarization mode dispersion value incident to one end side of a measured object while rotating a polarization plane of the light, and detecting at least one of a maximum value and a minimum value of a polarization mode dispersion value generated from a spectrum of a linearly polarized beam emitted from the other side of the measured object following the rotation of the polarization plane of the light; and
calculating a polarization mode dispersion value of the measured object by using any two values among the predetermined polarization mode dispersion value, the maximum value of the polarization mode dispersion value, and the minimum value of the polarization mode dispersion value.
Further, in order to achieve the above object, according to the present invention,
(2) there is provided a polarization mode dispersion measuring method described in (1), wherein
the step of detecting at least one of a maximum value and a minimum value of the polarization mode dispersion value detects the two of the maximum value and the minimum value of the polarization mode dispersion value, and
the step of calculating a polarization mode dispersion value of the measured object calculates the value of the polarization mode dispersion of the measured object, based on a difference between the maximum value of the polarization mode dispersion value and the minimum value of the polarization mode dispersion value.
Further, in order to achieve the above object, according to the present invention,
(3) there is provided a polarization mode dispersion measuring method described in (1), wherein
the step of detecting at least one of a maximum value and a minimum value of the polarization mode dispersion value detects the maximum value of the polarization mode dispersion value, and
the step of calculating a polarization mode dispersion value of the measured object calculates the value of the polarization mode dispersion of the measured object, based on a difference between the maximum value of the polarization mode dispersion value and the predetermined polarization mode dispersion value.
Further, in order to achieve the above object, according to the present invention,
(4) there is provided a polarization mode dispersion measuring method described in (1), wherein
the step of detecting at least one of a maximum value and a minimum value of the polarization mode dispersion value calculates the minimum value of the polarization mode dispersion value, and
the step of calculating a polarization mode dispersion value of the measured object calculates the value of the polarization mode dispersion of the measured object, based on a difference between the minimum value of the polarization mode dispersion value and the predetermined polarization mode dispersion value.
Further, in order to achieve the above object, according to the present invention,
(5) there is provided a polarization mode dispersion measuring system comprising:
a light source (21) for emitting a light having substantially uniform spectrum;
a first polarizer (22) for extracting a linearly polarized beam from the light emitted from the light source;
a reference object (23) having a predetermined polarization mode dispersion value, for emitting a polarization mode dispersion to the linearly polarized beam incident to one end side and extracted by the first polarizer, and emitting a light from the other end side;
a polarization plane rotor (24) for rotating a polarization plane of the light emitted from the reference object, and making the light incident to one end side of a measured object;
a second polarizer (25) for extracting a linearly polarized beam from a light emitted from the other end side of the measured object; and
an optical spectrum analyzer (26) for detecting a spectrum of a light emitted from the second polarizer, wherein
at least one of a maximum value and a minimum value of a polarization mode dispersion value generated following the rotation of the polarization plane of the light is obtained, from the spectrum of the linearly polarized beam emitted from the other end side at one end side of the measured object detected by the optical spectrum analyzer, and
the polarization mode dispersion value of the measured object is obtained by using optional two values among the predetermined polarization mode dispersion value, the maximum value of the polarization mode dispersion value, and the minimum value of the polarization mode dispersion value.
Further, in order to achieve the above object, according to the present invention,
(6) there is provided a polarization mode dispersion measuring system described in (5), wherein
both the maximum value and the minimum value of the polarization mode dispersion value generated following the rotation of the polarization plane of the light are obtained, from the spectrum of the linearly polarized beam emitted from the other end side at one end side of the measured object detected by the optical spectrum analyzer, and
the value of the polarization mode dispersion of the measured object is obtained based on a difference between the maximum value of the polarization mode dispersion value and the minimum value of the polarization mode dispersion value.
Further, in order to achieve the above object, according to the present invention,
(7) there is provided a polarization mode dispersion measuring system described in (5), wherein
the maximum value of the polarization mode dispersion value generated following the rotation of the polarization plane of the light is obtained, from the spectrum of the linearly polarized beam emitted from the other end side at one end side of the measured object detected by the optical spectrum analyzer, and
the value of the polarization mode dispersion of the measured object is obtained based on a difference between the maximum value of the polarization mode dispersion value and the predetermined polarization mode dispersion value.
Further, in order to achieve the above object, according to the present invention,
(8) there is provided a polarization mode dispersion measuring system described in (5), wherein
the minimum value of the polarization mode dispersion value generated following the rotation of the polarization plane of the light is obtained, from the spectrum of the linearly polarized beam emitted from the other end side at one end side of the measured object detected by the optical spectrum analyzer, and
the value of the polarization mode dispersion of the measured object is obtained based on a difference between the minimum value of the polarization mode dispersion value and the predetermined polarization mode dispersion value.
Further, in order to achieve the above object, according to the present invention,
(9) there is provided a polarization mode dispersion measuring system comprising:
a light source (21) for emitting a light having substantially uniform spectrum;
a first polarizer (22) for extracting a linearly polarized beam from the light emitted from the light source;
a reference object (23) having a predetermined polarization mode dispersion value, for emitting a polarization mode dispersion to the linearly polarized, beam incident to one end side and extracted by the first polarizer, and emitting a light from the other end side;
a polarization plane rotor (24) for rotating a polarization plane of the light emitted from the reference object, and making the light incident to one end side of a measured object;
a second polarizer (25) for extracting a linearly polarized beam from a light emitted from the other end side of the measured object;
spectrum detecting means (30) for receiving the light extracted by the second polarizer, automatically analyzing the spectrum of the light, and detecting a wavelength distance between adjacent peaks (or between adjacent bottoms) of the spectrum, a first peak wavelength xcex1, and a last peak wavelength xcex2; and
PMD calculating means (31) for calculating a PMD value P (a differential group delay time xcex94xcfx84) by automatically calculating the following expression from values detected by the spectrum detecting means:
P=k(nxe2x88x921)xc2x7xcex1xc2x7xcex2/(Cxc2x7xcex94xcex) 
where, k represents a mode coupling coefficient, C represents an optical speed, n represents a number of peaks, and xcex94xcex=xcex2xe2x88x92xcex1.